Method for operating an electrohydraulic unit
Patent Information
- Application Number
- EP2023727520
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-05-25
- Filing Date
- 2023-05-17
- Publication Date
- 2025-04-09
AI Technical Summary
Electro-hydraulic units face issues with machine downtime and excessive wear due to impermissible operating points, particularly in pressure-volume flow operations, where pressure is controlled using speed as a manipulated variable, leading to potential thermal overload and mechanical damage.
A method for operating electro-hydraulic units that adjusts the displacement volume of the hydraulic pump between two values, monitoring actual pressure and speed to ensure permissible operating points, and implementing protective mechanisms such as limiting torque, pressure change rates, and acceleration to prevent damage, using a computing unit to control the electric drive and pump.
This method increases pump availability, prevents downtime, and reduces wear by maintaining permissible operating points, avoiding thermal overload and mechanical damage, while maintaining control dynamics and energetic efficiency.
Smart Images
Figure 1.1
Abstract
Description
[0001] Method for operating an electro-hydraulic unit
[0002] Description
[0003] The present invention relates to a method for operating an electro-hydraulic unit.
[0004] Background of the invention
[0005] The electro-hydraulic units underlying the invention (so-called motor-pump units) have a hydraulic pump with variable displacement per working cycle (so-called hydraulic displacement machine, e.g., axial piston machine), which is driven by an electric drive (motor) with variable speed. The displacement can be continuously or stepwise adjustable. During operation of such electro-hydraulic units, the volume flow and / or the discharge pressure (i.e., pressure difference between inlet and outlet) are usually regulated by appropriately adjusting the speed and displacement of the hydraulic pump. This means that such electro-hydraulic units fundamentally have two degrees of freedom in control.
[0006] DE 102007 007 005 A1 discloses an electrohydraulic control system with an adjustable fluid pump and a variable-speed electric drive. A pressure sensing device allows the fluid pressure to be measured. The main control loop of this electrohydraulic control system has a speed actuator of the electric drive as the actuator. The main control loop can be used to control the fluid pressure and thus, for example, a force exerted by a cylinder or a downstream control variable such as a position or speed. A displacement actuator of the fluid pump is controlled by a secondary control chain depending on the measured fluid pressure.A particularly simple embodiment provides that the auxiliary control chain comprises a control device, in particular a two-point controller, by means of which the displacement volume of the adjustable fluid pump can be switched between a predetermined minimum value and a maximum value depending on a predetermined switching condition.
[0007] Disclosure of the invention
[0008] According to the invention, a method for operating an electro-hydraulic unit, as well as an electro-hydraulic unit, a computer program, and a data carrier are proposed with the features of the independent patent claims. Advantageous embodiments are the subject of the dependent claims and the following description.
[0009] The present invention leads to an increase in the availability of the pump in electro-hydraulic systems, prevents machine downtime and avoids or reduces damage or additional wear to the pump. In particular in the case of so-called pressure-volume flow operation (pQ) of a pump, in which the pressure is regulated using the speed as the manipulated variable and the volume flow is calculated as the product of the speed and displacement volume, it can be ensured that only operating points (i.e. pressure / speed combinations) which are within the permissible range are approached. If inadmissible operating points are approached due to the setpoint specification or configuration, this is prevented within the scope of the invention by switching the displacement volume, in particular by reducing it. With a small displacement volume, there are usually no operating point restrictions. For each hydraulic pump, permissible orInadmissible operating points are known in advance and, in particular, specified in a data sheet. They can be stored and monitored accordingly in the memory of the controlling processing unit.
[0010] To operate a variable-speed electro-hydraulic unit in which a hydraulic pump with a displacement volume that can be adjusted between a first larger and a second smaller displacement volume value per working cycle is driven by a variable-speed electric drive, a setpoint for the speed of the variable-speed electric drive is determined and specified to the electric drive. This can be done using a higher-level specification, e.g. a pressure and / or volume flow control, with the setpoint for the speed of the variable-speed electric drive being determined as a manipulated variable for a higher-level pressure control system, to which a setpoint for the delivery pressure across the hydraulic pump or, in general, a pressure at a specific point in the system is supplied as an input variable. The displacement volume is set to the first larger or the second smaller displacement volume value, expediently also using a higher-level specification, e.g.B. an operating strategy. For example, the torque load on the electric drive is reduced at a small displacement value, since the load torque is the product of the discharge pressure and the displacement. For example, at a small displacement, a high pressure can be maintained with a low torque. In particular, this prevents thermal overload of the electric motor and the electric drive's converter.
[0011] If the displacement volume now has the first larger displacement volume value, a monitoring process is carried out within the scope of the invention, in which an actual value of the discharge pressure across the hydraulic pump and an actual value of the speed of the hydraulic pump (or of the electric drive, since these are usually geared 1:1 or at least have a known gear ratio) are determined. It is determined whether a permissible operating point results from the actual value of the discharge pressure and the actual value of the speed of the hydraulic pump. If no permissible operating point results, the displacement volume is set to the second, smaller displacement volume value.
[0012] Preferably, by monitoring the current torque, ie by determining an actual value of a torque of the variable-speed electric drive, it can be determined whether the setting of the second smaller displacement volume value has taken place or was successful, since the torque changes as a result, in particular decreases.
[0013] Alternatively or additionally, the currently set displacement value can be determined using a computational model or model-based observer that incorporates existing measured variables such as speed, torque, drive current, and discharge pressure. This displacement value can also be used to determine whether the second, smaller displacement value has been set or was successful.
[0014] Another advantage is that the currently set displacement volume value can be considered as a controlled system property in pressure control. In particular, a system adaptation function is provided downstream of the actual controller, which outputs a manipulated variable. This function changes the manipulated variable (e.g., a speed setpoint) depending on the currently set displacement volume value. This eliminates the need to adapt controller parameters (coefficients such as proportional adjustment, reset time, etc.), and the required control dynamics can be maintained.
[0015] According to a preferred embodiment of the invention, when the setpoint speed of the variable-speed electric drive changes, the rate of change of the setpoint is limited to a maximum speed change rate. Changes to the setpoint speed result, in particular during normal operation, from the higher-level specification, but also as a result of the change in the displacement volume to the second, smaller displacement volume value, i.e., if the displacement volume is reduced, the speed must be increased to maintain the volume flow or pressure. The change in the setpoint speed occurs with a defined, limited angular acceleration. This allows further component protection, in particular pump protection, to be achieved. The speed change rate is, in particular, a rotational acceleration and can be expressed in [angle / time 2] must be specified. Limiting the acceleration / deceleration of the pump rotation can prevent mechanical damage to internal components. On the system side, high acceleration can lead to the formation of a local vacuum in the hydraulic fluid, as the fluid is dynamically unable to follow the movement of the pump rotor. This can lead to cavitation. A pump data sheet may list acceleration and deceleration limits.
[0016] According to a preferred embodiment of the invention, in the pressure control, a change rate of the setpoint pressure is limited to a maximum pressure change rate and / or the setpoint itself is limited to a maximum pressure value (e.g., the nominal pressure of the pump). This represents two further advantageous component protection mechanisms to protect the pump from overloads or damage. If the setpoint pressure change rate is already limited (i.e., on the input side), this automatically also leads to a time delay at the output, i.e., in particular, a speed change rate, and thus also has the aforementioned advantages. The pressure change rate can be specified, in particular, in [pressure / time]. Since exceeding the maximum pressure value may be permissible for a short time depending on the design and layout, limiting the setpoint pressure to a maximum pressure value preferably includes, before limiting the setpoint pressure, i.e.,before regulating, a permissible overload time is first waited for. According to a preferred embodiment of the invention, the setpoint value of the speed of the variable-speed electric drive is limited to a minimum speed if the actual value of the discharge pressure corresponds at most to a minimum pressure value. Usually, for energetic and dynamic reasons, the hydraulic pump should be operated with the largest possible displacement volume, i.e. in this case, the displacement volume should have the first larger displacement volume value. However, this means a large hydraulic torque, so that particularly low or even negative speeds (e.g. < 200 rpm) at high pressures (above a threshold value of e.g. 5 bar or 50 bar) are only permissible for short periods (e.g. a few seconds) (it goes without saying that the respective values depend on the specific pump and are to be understood purely as examples).As a further component protection mechanism, a minimum pressure value and a minimum speed can be monitored to prevent cavitation of the pump. If the speed can not be reduced any further due to the limitation, the displacement is reduced (and the speed increased accordingly) in order to maintain the flow rate. The limitation can be implemented immediately or with a time delay, i.e. a brief drop below the minimum speed can be permitted if necessary. However, if in the second case it becomes apparent that the permissible time will be exceeded, the system switches to the small displacement, particularly taking the swivel time into account. Taking the swivel time into account means that the permissible time is not exceeded until the displacement has reached the small displacement value (and can then no longer be exceeded because a non-critical operating point exists).
[0017] A computing unit according to the invention, e.g. a control unit of an electro-hydraulic unit, is configured, in particular in terms of programming, to carry out a method according to the invention.
[0018] The implementation of a method according to the invention in the form of a computer program or computer program product with program code for carrying out all method steps is also advantageous, as this entails particularly low costs, particularly if an executing control unit is also used for other tasks and is therefore already present. Suitable data carriers for providing the computer program are, in particular, magnetic, optical, and electrical storage devices, such as hard disks, flash memories, EEPROMs, DVDs, and others. Downloading a program via computer networks (Internet, intranet, etc.) is also possible. Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.
[0019] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.
[0020] The invention is illustrated schematically in the drawing using an embodiment and is described in detail below with reference to the drawing.
[0021] Character description
[0022] Figure 1 shows schematically a preferred embodiment of a variable-speed electro-hydraulic unit according to the invention.
[0023] Detailed description of the drawing
[0024] In Figure 1, a preferred embodiment of a variable-speed electro-hydraulic unit according to the invention is schematically shown and designated overall by 10. The electro-hydraulic unit 10 has an adjustable hydraulic pump 20 (which in the present example can pump in two directions), a variable-speed electric drive
[0025] 30 and a computing unit 40 for operating the electro-hydraulic unit.
[0026] The hydraulic pump 20 is designed as an axial piston pump with a pivot angle a for a swash plate or inclined axis that can be adjusted between two positions. This allows the displacement volume to be adjusted between a first larger and a second smaller displacement volume value. A control signal from the computing unit 40 to the axial piston pump 20 can be used for this purpose. The control signal can, for example, be a control value for a so-called pilot valve 21 on the pump.
[0027] The electric drive is designed as a so-called standard motor, which uses a synchronous servo motor
[0028] 31 and a frequency converter 32. In principle, asynchronous motors can also be used. The speed n of the asynchronous motor 31 is variable. For this purpose, a target speed is specified by the computing unit 40 to the electric drive 30. The actual speed can be determined via an angle sensor. However, control without feedback is also possible.
[0029] The arithmetic unit 40, to which a target delivery pressure ps and / or a target volume flow Qs are supplied, is used to control the pump 10. The arithmetic unit 40 can be designed as a so-called electric drive controller and, within the scope of the invention, determines the speed n as a manipulated variable. The arithmetic unit can also be separate from the drive controller and superordinate to it, communicating with it via an interface (speed setpoint, actual speed value, etc.). The actual delivery pressure is expediently determined via a sensor. This enables pressure control.
[0030] A change in displacement volume during operation can occur, for example, based on an operating strategy. For example, the computing unit 40 can determine whether the electric drive is in full-load operation or partial-load operation, e.g., by monitoring the electrical current in order to determine the electric drive torque supplied by the electric drive 30 and compare it with the maximum possible electric drive torque for the current speed, or by monitoring the speed. In particular, a threshold comparison can be provided, whereby an electric drive torque below a threshold value (of, for example, 75%) or a speed below a threshold value is recognized as partial load. The speed threshold is expediently specified depending on the intended use.
[0031] During full-load operation, a control signal for the swivel angle of pump 20 is output, which results in the first, larger displacement value. During partial-load operation, a control signal for the swivel angle of pump 20 is output, which results in a second, lower displacement value. In particular, the dependence of the second, lower displacement on the speed can be obtained from a characteristic map or similar in the computing unit.
[0032] The displacement volume can expediently be adjusted by controlling the pilot valve or control valve 21. A switching valve (or a proportional valve) can be used for this purpose. According to a preferred embodiment of the invention, different protective mechanisms are used to protect the electro-hydraulic unit and its components from damage. For this purpose, an actual value of the delivery pressure across the hydraulic pump and an actual value of the speed n of the asynchronous motor 31 (as the speed of the hydraulic pump) are determined (in particular) when the displacement volume has the first larger displacement volume value, and from this it is determined whether a permissible operating point exists. If no permissible operating point exists (because in particular the speed is too low and / or the pressure is too high), the displacement volume is set to the second, smaller displacement volume value. To maintain the pressure ps orAs the flow rate Qs increases, the speed is increased accordingly, leaving the impermissible operating point. As explained above, there are usually no operating point restrictions for small displacement volumes.
[0033] Further advantageous protective mechanisms include limiting a rate of change of the setpoint value of the speed of the variable-speed electric drive (30) to a maximum speed change rate, limiting a rate of change of the setpoint value of the delivery pressure via the hydraulic pump (20) to a maximum pressure change rate, limiting the setpoint value of the delivery pressure via the hydraulic pump (20) to a maximum pressure value, if necessary after a permissible overload time has been exceeded, limiting the setpoint value of the speed of the variable-speed electric drive (30) to a minimum speed if the actual value of the delivery pressure corresponds at most to a minimum pressure value, if necessary after a permissible overload time has been exceeded.
[0034] The present invention increases pump availability in electro-hydraulic systems, prevents machine downtime, and avoids or reduces damage or excessive wear to the pump.
Claims
Claims 1. Method for operating a variable-speed electro-hydraulic unit (10) in which a hydraulic pump (20) whose displacement volume is adjustable between a first larger and a second smaller displacement volume value per working cycle is driven by means of a variable-speed electric drive (30), comprising, when the displacement volume has the first larger displacement volume value: Determining a target value for the speed of the variable-speed electric drive (30), determining an actual value for the delivery pressure across the hydraulic pump (20), determining an actual value for the speed of the hydraulic pump (20), Determine whether a permissible operating point results from the actual value of the delivery pressure and the actual value of the speed of the hydraulic pump (20), and Setting the second smaller displacement volume value if no permissible operating point results from the actual value of the delivery pressure and the actual value of the speed of the hydraulic pump (20).
2. The method according to claim 1, further comprising: Determining an actual value of the torque of the variable speed electric drive (30) and determining whether the setting of the second smaller displacement volume value has taken place, based on the determined actual value of the torque.
3. Method according to claim 1 or 2, further comprising: Limiting the rate of change of the setpoint of the speed of the variable-speed electric drive (30) to a maximum speed change rate.
4. Method according to one of the preceding claims, wherein the setpoint of the speed of the variable-speed electric drive (30) is determined as a control variable of a higher-level pressure control, to which a setpoint of the delivery pressure above the hydraulic pump (20) is supplied as an input variable.
5. The method of claim 4, further comprising: Limiting the rate of change of the setpoint of the delivery pressure across the hydraulic pump (20) to a maximum pressure change rate.
6. The method of claim 4 or 5, further comprising: Limiting the setpoint of the delivery pressure above the hydraulic pump (20) to a maximum pressure value.
7. The method of claim 6, wherein limiting the setpoint of the delivery pressure above the hydraulic pump (20) to a maximum pressure value comprises: Waiting for a permissible overload time before limiting the target value of the delivery pressure.
8. Method according to any one of claims 4 to 7, further comprising: taking into account the currently set displacement volume value as a path property of the pressure control for changing the manipulated variable.
9. Method according to any one of the foregoing claims, further comprising: Determining the currently set displacement volume value using a calculation model.
10. Method according to any of the foregoing claims, further comprising: Limiting the setpoint of the speed of the variable-speed electric drive (30) to a minimum speed when the actual value of the delivery pressure corresponds to at most a minimum pressure value.
11. Method according to claim 10, wherein limiting the setpoint of the speed of the variable-speed electric drive (30) to a minimum speed comprises: Waiting for a permissible overload time before limiting the setpoint of the speed of the variable-speed electric drive (30).
12. Computing unit (40) comprising a processor configured to perform the method according to any of the preceding claims.
13. Electro-hydraulic unit (10), comprising a hydraulic pump (20) adjustable in a displacement volume per working cycle between a first larger and a second smaller displacement volume value, a variable speed electric drive (30) and a computing unit (40) according to claim 12.
14. Computer program comprising instructions which, when executed by a computing unit of an electro-hydraulic power unit (10) according to claim 12, cause it to execute the method according to claims 1 to 9.
15. Computer-readable data carrier on which the computer program according to claim 14 is stored.